One of the greatest advantages you can have over your competition is the ability to find gold-making opportunities by yourself, before they become common knowledge. Always explore different markets, look for niches that can be exploited, and undiscovered markets. When you're in-game, make sure to keep tabs on these things and look for potential ways to make gold; consider the following:
Check your professions for odd items that many players may not be bothered with but for which there's demand in the Auction House. These can include items such as the Overcharged Capacitor made by Engineers that are required for two quests in Northrend.
When you are in remote parts of the world, check vendors for items of limited supply. You can tell an item is in limited supply if it has a count in parenthesis next to the item name in the vendor window.
When you are doing quests, especially quests for reputation, which require turn-ins, see if the items are...